(1) Field of the Invention
This invention relates to a process for sizing cellulose fiber products such as paper and paperboards with cellulose-nonreactive rosin-base materials, and more particularly to a process for sizing cellulose fiber products with pH neutral materials.
(2) Description of the Prior Art
Cellulose fiber products such as paper and paperboards are produced from an aqueous slurry, or furnish, of cellulose fibers containing sizing agents admixed therewith. These sizing agents generally comprise aqueous dispersions of rosin, especially fortified rosin, which is utilized to modify the surface of the paper to control water penetration. Such sizing is termed internal sizing and is an important step in the wet end operation of a paper machine.
Rosin, or rosin acid, itself has no affinity for cellulose fiber (i.e., rosin is cellulose-nonreactive) and must be anchored to the surface of the cellulose fiber with a cation. In the past, this has been done with the aluminum ion, normally derived from alum. The rosin acid size and aluminum ions do not react in solution but are co-deposited on the fiber surface, with the size held on the pulp fiber by electrostatic forces. At this point, the rosin acid size is not yet hydrophobic, but it becomes water-repellent after interaction with the alum in a subsequent heat curing step. The curing occurs when rosin size melts in the dryer section of the paper machine and the molten rosin acid spreads over the fiber surface and reacts with neighboring alum absorbed on it. The resulting aluminum rosinate is to a large extent responsible for the degree of water repellency of the paper product.
In the system described, the alum is effective as a precipitant only when the pH of the fibrous suspension is acid, typically around 4.5. This acidity has a detrimental corrosive action on paper making equipment, and results in the paper itself having an acid pH, which in the long term causes degradation of the paper. Moreover, the acid pH precludes the replacement of expensive titanium dioxide filler with less expensive but acid sensitive calcium carbonate.
It has been proposed in the art to use a cellulose-reactive size, such as a ketene dimer, which has an affinity for the cellulose in conjunction with a fortified rosin, as taught by Dumas in European Pat. Appln. No. 0 074 544, to achieve sizing at either acid or neutral pH either with or without alum. Of course, the alum system is preferred to involve both components in the sizing process.
Dumas, in U.S. Pat. No. 3,582,464, teaches the use of the acid anhydride form of rosin which, due to its affinity for cellulose, acts as a cellulose-reactive size material. This permits sizing in the absence of alum at acid or neutral pH, but the modified form of rosin is more expensive and, without rosin, the desired hydrophobicity is not achieved.
Also, it has been proposed in the art, particularly in U.S. Pat. Nos. 3,186,900, 3,248,353 and 3,966,654 to utilize cationic epichlorohydrin-containing resins as retention aids in the manufacture of paper. Although sizing has been shown with these systems at a neutral pH, the systems quite often involve the use of alum, nevertheless. The use of alum at a neutral pH is considered undesirable since it can result in the precipitation of aluminum hydroxide in the paper making equipment.